a) Field of the invention:
The present invention relates to a zoom lens system, and more specifically to a zoom lens system which is designed for use with cameras employing electronic image pickup devices.
b) Description of the prior art
Along with the recent trend to manufacture compact, light-weight domestic video cameras at low costs, efforts are being made to develop compact, light-weight photographic lens systems which can be manufactured at reduced cost.
As compact zoom lens systems which have simple compositions and designed for use with silver salt cameras, there are known, for example, the zoom lens systems proposed by Japanese Patent Kokai Publication No. Sho 55-62420 and Japanese Patent Kokai Publication No. Sho 56-123512.
Each of these conventional zoom lens systems consists, in the order from the object side, of a first lens unit having a negative refractive power as a whole and a second lens unit having a positive refractive power as a whole. This type of zoom lens system allows the optical performance thereof to be remarkably degraded by eccentricity of the lens elements composing the lens units since the axial ray is high on the second lens unit, especially at the tele position. The zoom lens system proposed by Japanese Patent Kokai Publication No. Sho 56-123512 adopts a cemented doublet in the second lens unit to prevent the lens component from being eccentric at the stage of assembly into a barrel.
Both of the conventional zoom lens systems are designed for use with film camera and it is necessary, for using these zoom lens systems with video cameras, to locate the exit pupils of the lens systems sufficiently far from the image surfaces. If the exit pupils of the zoom lens systems are not located sufficiently far from the image surfaces, rays will have large angles of incidence and be vignetted on the marginal portions of the image surface due to the structure inherent in the image pickup devices, thereby producing an undesirable effect to degrade sensitivity at the marginal portions of the image surfaces.